1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly to transmissive liquid crystal display devices having a cholesteric liquid crystal polarizing plate and a cholesteric liquid crystal color filter layer.
2. Discussion of the Related Art
Generally, liquid crystal display (LCD) devices operate using optical anisotropy and polarization properties inherent to liquid crystal molecules. Liquid crystal molecules have definite orientational alignment characteristics resulting from their thin and long shape. The orientational alignment of liquid crystal molecules can be controlled by applying an electric field to the liquid crystal molecules wherein, as the intensity of the applied electric field changes, the orientational alignment of the liquid crystal molecules also changes. The intensity of light incident the liquid crystal molecules can be selectively controlled to display images due to the aforementioned anisotropic optical properties of the liquid crystal molecules, wherein incident light becomes refracted due to the orientation of the liquid crystal molecules.
Active matrix LCD (AM-LCD) devices include thin film transistors (TFTs) and pixel electrodes, connected to the TFTs, arranged in a matrix pattern and are capable of images at a high resolution as well as moving images.
FIG. 1 illustrates a schematic perspective view of a related art LCD device.
Referring to FIG. 1, the related art LCD device 11 includes an upper substrate 5 (i.e., the color filter substrate) separated from a lower substrate 22 (i.e., the array substrate) by a layer of liquid crystal material 14. A black matrix layer 6 and a color filter layer 8 having red, green and blue sub color filters 8a, 8b, and 8c, respectively, formed on the upper substrate 5. A transparent common electrode 18 is formed on the color filter layer 8 and on the black matrix layer 6. The lower substrate 22 supports array lines such as gate lines 13, data lines 15, and switching elements “T” connected to respective ones of the gate and data lines 13 and 15, respectively. Pixel electrodes 17 are formed in a pixel region “P” of the lower substrate 22, defined by crossings of the gate and data lines 13 and 15, respectively. The pixel electrode 17 at the pixel region “P” is made of a transparent conductive material such as indium-tin-oxide (ITO), indium-zinc-oxide (IZO), or other material having high light transmittance characteristics. A light source such as a backlight unit (not shown) is disposed beneath the LCD device 11.
When gate signals are applied to switching elements “T”, data signals are applied to corresponding ones of the pixel electrodes 17. Likewise, when gate signals are not applied to the switching elements “T”, data signals are not applied to the corresponding ones of the pixel electrodes 17. Accordingly, the LCD device behaves as a light modulating device, modulating light provided by the backlight unit that passes through a plurality of optical films to display images.
The aforementioned related art LCD device, however, uses the light provided by the backlight unit in a relatively inefficient manner. For example, the aforementioned plurality of optical films generally include a pair of linear polarizing plates, transmitting a linear component of the light provided by the backlight unit in a single direction, and a color filter layer. Accordingly, less than about half of the light provided by the backlight unit is transmitted by the pair of linear polarizing plates, thereby reducing the brightness of the LCD device. Further, the aforementioned color filter layer is provided as an absorption type filter that greatly reduces the intensity of the light provided by the backlight unit, thereby further reducing the brightness of the LCD device.
To alleviate the problematic reduction in LCD device brightness, absorptive color filter layers having a high light transmittance characteristics have been used. However, as the light transmittance characteristics of absorptive color filter layers increase, their ability to generate light having high color purity decreases. Accordingly, there is a limit to which the light transmittance characteristics of absorptive color filter layers can be increased.
To solve the aforementioned problems of absorptive color filter layers in LCD devices, LCD devices using cholesteric liquid crystal color filter (CCF) layers have been researched and developed. CCF layers use selective reflection properties inherent to cholesteric liquid crystal (CLC) material from which they are formed to selectively reflect/transmit light within a predetermined wavelength range. The selective reflection/transmission properties of CCF layers are dependent upon a helical pitch of the CLC material from which they are formed. Accordingly LCD devices may include CCF layers made of CLC material having different helical pitches corresponding to predetermined pixel regions. By replacing absorptive color filter layers with CCF layers, the efficiency with which light generated by backlight units is used may be increased.
FIG. 2 illustrates a schematic cross-sectional view of a related art reflective LCD device including a related art CCF layer.
Referring to FIG. 2, first substrate 5 opposes and is spaced apart from second substrate 22. A first electrode 18 made of transparent conductive material is formed on an inner surface of the first substrate 5. A retardation film 30 and a linear polarizing film 32 are subsequently and successively formed on an outer surface of the first substrate 5. A cholesteric liquid crystal color filter (CCF) layer 24 is formed on an inner surface of the second substrate 22 by depositing and patterning molecules of cholesteric liquid crystal (CLC). While not shown, the CCF layer 24 includes sub cholesteric liquid crystal color filters (sub CCFs), capable of reflecting light at wavelengths corresponding to red, green, and blue colors. A second electrode 17 made of transparent conductive material is formed on the CCF layer 24. A light-absorbing layer 34, made of a light absorbing material such as a polymer, is formed on an outer surface of the second substrate 22. Lastly, a layer of liquid crystal material 14 is interposed between the first and second electrodes 18 and 17.
As mentioned above, the CCF layer 24 is formed using CLC material that selectively reflects/transmits right-handed (or left handed) circularly polarized light having a wavelength range in accordance with a helical pitch of the CLC material. Accordingly, the wavelength range of the light selectively reflected/transmitted by the CCF layer 24 may be adjusted by adjusting the pitch of the CLC. A central wavelength of the wavelength band corresponds to one of red, green and blue colors. As a result, the CCF layer 24 can selectively reflect red, green and blue light by adjusting a pitch of the CLC wherein the reflected light is subsequently transmitted within a corresponding red, green, or blue pixel region by recycling the reflected light. More specifically, the light reflected by the CCF layer has a higher intensity than light selectively transmitted by the aforementioned absorptive color filter layer. Accordingly, color purity and a color reproducibility may be improved.
However, even though the aforementioned related art reflective LCD device, including the CCF layer, yields improved color purity and improved color reproducibility, the related art reflective LCD device uses light provided by the backlight unit inefficiently because all light, except for the reflected light, is absorbed by the light-absorbing layer 34. As a result, the related art reflective LCD device has poor brightness and a poor contrast ratio.